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Pure Samples of Individual Conformers The Separation of Stereoisomers of Complex Molecules Using Electric Fields.

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Communications
DOI: 10.1002/anie.200902650
Separation of Isomers
Pure Samples of Individual Conformers: The Separation of Stereoisomers of Complex Molecules Using Electric Fields
Frank Filsinger, Jochen Kpper,* Gerard Meijer, Jonas L. Hansen, Jochen Maurer,
Jens H. Nielsen, Lotte Holmegaard, and Henrik Stapelfeldt*
Many complex molecules have multiple structural isomers;
that is, multiple local minima on their potential energy
surface. About twenty-five years ago, it was observed that
multiple conformers of tryptophan are present even at the low
temperatures of a few Kelvin in a supersonic jet.[1] These
conformers have been studied extensively since then with
sophisticated spectroscopic techniques. Individual conformers can be identified from their distinct electronic[1, 2] or
microwave[3] spectra. Information on the conformational
structures can be obtained using microwave[4] or multipleresonance infrared spectroscopy, for example.[5, 6] In similar
experiments it was even possible to obtain information on the
barriers separating the conformers.[7]
The preparation of spatially separated conformers would
provide unique possibilities for advanced further investigations. The chemical properties of the individual species and
their differences could be directly studied in reactive scattering experiments. Such pure samples would also enable a new
class of experiments, such as electron[8] and X-ray diffraction[9, 10] or tomographic imaging[11] experiments of complex
molecules in the gas phase. Molecular-frame photoelectron
angular distributions, ultrafast time-resolved photoelectron
spectroscopy, and ultrafast dynamics studies[12] would also
benefit from the availability of these pure samples. For
charged species, the separation of molecules with different
shapes has been demonstrated by utilizing ion mobility in drift
tubes.[13, 14] For neutral molecules, the abundance of the
conformers in molecular beams can be partly influenced by
selective over-the-barrier excitation in the early stage of the
expansion[15] or by changing the carrier gas.[16]
Herein, we demonstrate that electrostatic deflection, a
classic molecular beam manipulation method that dates back
to the 1920s,[17, 18] allows the spatial separation of the con[*] F. Filsinger, Dr. J. Kpper, Prof. Dr. G. Meijer
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4–6, 14195 Berlin (Germany)
Fax: (+ 49) 30-8413-5892
http://www.fhi-berlin.mpg.de/mp/jochen
E-mail: jochen@fhi-berlin.mpg.de
J. L. Hansen, Prof. Dr. H. Stapelfeldt
Interdisciplinary Nanoscience Center (iNANO), University of Aarhus
8000 Aarhus C (Denmark)
http://www.chem.au.dk/en/staff/index.html?action = 3&person_id = 31
E-mail: henriks@chem.au.dk
J. Maurer, L. Holmegaard, Prof. Dr. H. Stapelfeldt
Department of Chemistry, University of Aarhus
J. H. Nielsen
Department of Physics and Astronomy, University of Aarhus
6900
formers of a neutral molecule when it is applied to intense
beams of rotationally cold molecules produced by a state-ofthe-art pulsed supersonic expansion source.[19] The idea of
exploiting electrostatic deflection to separate quantum states
was already conceived by Stern in 1926 for light diatomic
molecules,[20] and these ideas were recently extended to
proposals for the separation of conformers of large molecules.[21, 22]
Polar molecules experience a force in an inhomogeneous
electric field. This force is due to the spatial variation in the
!
potential energy of the molecules, and is given by F =
!
meff·r E. The effective dipole moment meff of a molecule in a
given quantum state is the negative gradient of the potential
energy with respect to the electric field strength E. This force
has been used to decelerate small molecules in a supersonic
jet to a standstill and subsequently trap them.[23] Similarly,
large neutral molecules have been deflected,[22, 24–26]
focused,[27] and decelerated.[28] Passing polar molecules
through a strong inhomogenous electric field will spatially
disperse them according to their effective dipole moment. In
particular, the conformers of a specific biomolecule all have
the same mass m, but differ by the relative orientations of
their functional groups. Typically, these functional groups
have large local dipole moments associated with them, and
the vectorial sum of these local dipole moments largely
determines the overall dipole moment of the molecule.[29]
Herein, we show that the resulting different overall dipole
moments of the conformers can be exploited to select
individual conformers using an electrostatic deflector.
The cis and trans conformers of 3-aminophenol (Figure 1)
are used herein as prototypical structural isomers of complex
molecules. From the precisely known rotational constants and
dipole moments,[29] the energies of the rotational states of cis3-aminophenol and trans-3-aminophenol are calculated as a
function of electric field strength. Figure 1 shows the resulting
Stark curves for the lowest rotational states of both species.
From Figure 1, it is obvious that the effective dipole moments
meff of the states of cis-3-aminophenol are considerably larger
than for trans-3-aminophenol, and therefore, a strong spatial
separation of the conformers is expected. The results on the
separation of the cis and trans conformers of 3-aminophenol
are complementary to our previous experiments on the
separation of the same species using an ac (alternating
current) focusing device,[27] and we will briefly discuss the
merits of the individual techniques in the Summary.
The experimental setup is shown in Figure 2, and a
detailed description is given elsewhere.[22, 26] 3-aminophenol
(Sigma–Aldrich, 98 %), seeded in 90 bar of helium, is released
from a pulsed valve into high vacuum. The molecular beam is
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 6900 –6902
Angewandte
Chemie
Figure 1. Molecular structures, dipole moments m, and energies W of
the lowest rotational states of cis- and trans-3-aminophenol as a
function of the electric field strength E.
Figure 2. The experimental setup, consisting of the molecular beam
source, the electrostatic deflector, and a photoionization time-of-flight
mass spectrometer. Inset: a section through the deflector with a
contour plot of the electric field strength.
collimated using two skimmers before it enters the 15 cm-long
electrostatic beam deflector. Inside the deflector an inhomogeneous electric field with a nearly constant gradient over a
large area around the molecular beam axis is created. The
deflector is mounted such that the deflection occurs vertically,
and the 3-aminophenol molecules are deflected upwards.
After passing the deflector, the molecular beam enters the
target area, where it is crossed by a focused pulsed dye laser,
which allows conformer-selective detection.
The spatial separation that is obtained is demonstrated by
the conformer-selective deflection profiles in Figure 3. The
density of a specific conformer[30] is measured as a function of
the height y of the focused detection laser by a speciesselective resonance-enhanced multiphoton ionization
(REMPI) setup. When high voltages are applied to the
deflector, both conformers are deflected upwards. However,
the shift is considerably larger for the more polar cis-3aminophenol, and above y = 1 mm a pure sample of cis
conformers exists. Additionally, owing to the low internal
temperature (ca. 1 K) of the initial molecular beam, the
population of cis-3-aminophenol can be almost completely
depleted for heights smaller than y = 0.75 mm, and an
Angew. Chem. Int. Ed. 2009, 48, 6900 –6902
Figure 3. Molecular beam intensity I as a function of the vertical
position y of the detection laser for cis- and trans-3-aminophenol.
Experimental data are given by symbols, and simulations by solid
lines. The vertical profiles of the undeflected beams of cis- and trans-3aminophenol are shown as circles (dark blue) and diamonds (orange),
respectively. Squares (light blue) and triangles (red): corresponding
deflection profiles with high voltage (10 kV) applied to the deflector.
Inset: the fractional population Irel of the cis conformer, which is
obtained by dividing the cis intensity by the sum of the intensities of
cis and trans at the respective height y; the horizontal line indicates the
value in the original beam.
almost pure sample of trans-3-aminophenol is obtained there.
In the inset of Figure 3, the fractional intensity of the cis
conformer in the deflected molecular beam is shown. It is
clear that the fraction of cis-3-aminophenol in the probed
sample can be continuously tuned by scanning the height of
the probe laser focus, and importantly, at heights above the
cut-off of the trans-3-aminophenol beam profile at y = 1 mm,
the density of the cis conformers is still comparable to the
density in the free jet.
The clean separation of the two conformers is also
confirmed by the vibrationally resolved REMPI spectrum
(Figure 4). The spectrum measured in the deflected part of
the molecular beam (y = 1.15 mm) only contains bands arising
from cis-3-aminophenol. In contrast, the spectrum measured
in the depleted beam (y = 0.75 mm) shows only features that
Figure 4. UV spectra of 3-aminophenol for the original beam (black),
the deflected ensemble (blue), and the depleted beam (red). The
spectral signatures and the complete discrimination of the individual
conformers are thus demonstrated.
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6901
Communications
can be assigned to the trans conformer. Of course, these
conformer-specific UV spectra can also be obtained using
double-resonance spectroscopy.[31] However, herein we obtain
these spectroscopic signatures using a single laser to demonstrate the spatial separation of the conformers. Whereas the
trans-3-aminophenol samples are still overlaid with seed gas
from the molecular beam, the deflected cis-3-aminophenol
sample is completely separated from trans-3-aminophenol
molecules and the atomic seed gas. It should be pointed out
that the lowest rotational states of cis-3-aminophenol, which
are the most polar states, are deflected the most. These states
can be aligned and oriented extremely well, providing the
possibility for strong confinement of the rotational motion of
the molecules.[22, 26] It can be envisioned that this unprecedented control over the molecular ensemble can be exploited
to perform stereospecific experiments on conformer- and
state-selected samples.
In summary, we have demonstrated the spatial separation
of individual conformers of 3-aminophenol using inhomogeneous electric fields using two different approaches, namely
the deflection setup presented herein and the dynamic
focusing setup demonstrated previously.[27] Both methods
could be applied to many other molecules, including systems
containing more than two conformers, as long as the temperature of the original beam is low enough.[22] However, there
are profound differences between the two approaches:
Whereas the ac focuser allows in principle all the conformers
to be addressed individually, the deflector generally only
separates off the most polar conformer. However, we have
demonstrated herein that pure samples of other conformers
can be created when appropriate experimental conditions are
chosen. Moreover, in experiments with the focusing selector,
the molecules are confined to the beam axis, resulting in a
background of non-polar molecules and atomic seed gas. In
contrast, the deflector separates the lowest rotational quantum states of the most polar conformer from any other
species. Last but not least, it should be noted that the
deflection experiment is technically considerably easier: the
mechanical setup is much simpler and no high-voltage
switching is required.
Received: May 18, 2009
Published online: August 13, 2009
.
Keywords: conformation analysis ·
dipole moment mass selection · isomers · laser spectroscopy ·
mass spectrometry
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